Workflow still matters

As we said yesterday, workflow still matters, and I'm catching this stuff again in the Realworld(tm), trying to clear concepts and how-tos in these (not so) new componentes of .NET. Workflow Foundation is one of these, and I'll try to talk also about WCF, WPF/Silverlight, etc. from here and beyond.

So I'll start with one simple but interesting question. Let's assume you need to develop some business logic in .NET... there any reason for not using Workflow Foundation?

Note that I'm not asking about doing all-or-nothing using WF. It's just a matter of searching for a reason for not using it as a part of the solution.

I'll try to look for the anwser for myself but, anyways, comments/discussions are welcome!